The diesel particulate filter (hereinafter referred to as DPF), which traps particulate matter included in exhaust gas discharged from a diesel engine, is characterized in that the greater the accumulated amount of particulate matter, the greater the exhaust flow resistance, resulting in an increased differential pressure between upstream and downstream of the DPF.
To eliminate the accumulated particulate matter, so-called regeneration of the DPF is carried out. According to such regeneration, it is determined that the accumulated amount of particulate matter has reached a reference trapping amount when the DPF pressure is higher than a predetermined pressure, and the particulate matter accumulated in the DPF is then burned, allowing the DPF to resume successfully trapping particulate matter.
However, the particulate matter is not evenly accumulated in the DPF and tends to be centrally accumulated in and around the inlet of the DPF. The particulate matter accumulated in this area tends not to be easily burned as opposed to that accumulated further downstream the DPF. This emanates from the fact that combustion heat generated in the upstream region of the DPF is available for burning the particulate matter accumulated in the DPF, while on the other hand, only the heat of the exhaust gas can be used to burn particulate matter in and around the inlet.